The propagation of interstellar jets is examined by means of numerical modeling. The jets result from the injection of an overpressured supersonic beam of gas into the interstellar medium, and have been followed time dependently up to distances of 0.2 pc from their injection point while preserving a high degree of collimation. As the jets propagate, their internal structure undergoes a rapid and complex evolution. The working surface shocks (incident, reflected and Mach disk) for example, almost periodically evolve into crossing shocks that recollimate the injected matter, while new sets of shocks form at the head of the jet. This internal evolution also causes important changes in the shape of the leading bow shock, while the earliest established crossing shocks approach a steady state location within the jet structure. The paper focuses on the rapid jet evolution, the drastic changes at the head of the jet, the formation and appearance of crossing shocks, and the relation of the latter to the physical conditions of the background medium.